Ultrasonic doppler blood flowmeter

ABSTRACT

An ultrasonic Doppler blood flowmeter measures the velocity of blood flow at an arbitrary position of the body of a subject by making use of Doppler shift of an ultrasonic echo signal. The echo signal including Doppler shift from the blood flow of the body is detected in phase on the basis of standard reference signal, and the distance to an intended position is extracted from the phase detected signal and is integrated, and the direct-current component or ultraflow frequency component of the Doppler shift signal obtained from the integrator means is fed back negatively to the integrator means. As a result, the echo signal components of the body tissues are removed, and the frequency of blood flow only can be analyzed, so that an accurate blood flow information may be obtained.

This application is a divisional of application Ser. No. 865,012, filed5/20/86.

BACKGROUND OF THE INVENTION

This invention relates to an ultrasonic Doppler blood flowmeter used inthe measurement of the velocity of blood flow at an arbitrary positionwithin the body of a subject in the medical field.

DESCRIPTION OF THE PRIOR ART

Recently, ultrasonic Doppler blood flowmeters have been used in themedical field for use in the cardiovascular system. Its operationdepends on the principle that the ultrasonic wave transmitted into thebody undergoes a frequency shift due to the Doppler effect which occurswhen the wave is reflected by a moving substance such as blood flow, andby indicating the Doppler shift frequency corresponding to the rate ofblood flow, the distribution of the rate of blood flow in the body canbe easily observed from the surface of the body. Such an ultrasonicDoppler blood flowmeter is disclosed, for example, in IEEE, Trans. S.U.,SU-17 (3), p.170, 1970.

Referring now to FIG. 1, the conventional ultrasonic Doppler bloodflowmeter is described below. In this diagram, numeral 101 denotes anultrasonic transmission and reception means (probe) which transmitsultrasonic pulses into the body from an ultrasonictransmission/reception plane 101a, and receives an echo signal which isreflected due to the difference in the acoustic impedances, generallythe probe 101 is made of a piezoelectric material. Numeral 102 is adrive circuit which generates the drive voltage for generatingultrasonic pulses to be transmitted from the probe 101 at the timing ofan external trigger and at the frequency of an external clock; the drivecircuit 102 drives the probe 101 element; 103 is a transmission timingcircuit which provides the timing for the drive circuit 102 to generatea drive voltage used as a trigger; element 104 is a phase detector usedto detect the phase of the echo signal received by the probe 101;element 105 is a reference signal generating circuit to serve as thereference of the frequency and phase of reference signal when detectingthe phase of the transmission signal and echo signal of the drivecircuit 102 by means of the phase detector 104; element 106 is a gatesignal generating circuit which generates a gate signal at the timingcorresponding to the propagation time of the ultrasonic wave up to thetransmission/reception plane of the probe 101 and the desired position;element 107 is an analog switch which causes the phase signal detectedby the phase detector 104 to pass during the interval of the gate signalgenerated by the gate signal generating circuit 106; element 108 is anintegrating circuit which integrates the phase signal passing throughthe analog switch 107, and thereby determines the sum of phase signals,and repeats the transmission and reception to obtain a Doppler shiftsignal; element 110 is a sample and hold circuit which holds theintegrated result for resetting until the result of next integration isobtained prior to integration by the integrating circuit 108; element111 is a high pass filter which removes signals of less than severalhundred hertz from the Doppler shift signal obtained in the integratingcircuit 108; element 112 is a frequency analyzer which analyzes thefrequency of the Doppler shift signal passing through the high passfilter 111; and element 113 is a display unit which indicates the resultof frequency analyzer 112.

While referring to the time chart shown in FIG. 2, its action isdescribed below. In the transmission timing circuit 103, trigger signalsT of constant or arbitrary intervals are generated (a), and are suppliedto the drive circuit 102. The drive circuit 102 drives the probe 101 bythe trigger signals T in drive pulse TX (b), and the ultrasonic pulsesare transmitted from this probe 101 into the body. The ultrasonic pulsespropagate within the body, and are reflected by the parts differing intheir acoustic impedance, and return to the probe 101, being received asecho signals E (c). The echo signals E are obtained at a delay timet_(d) from t_(o) of trigger signal T, corresponding to the reciprocalpropagation time of the ultrasonic wave from the transmission/receptionplane 101a of the probe 101 to the reflecting point of the transmittedultrasonic pulse, and since the echo signals from a moving reflectingbody always vary in the reciprocal propagation time, t_(d) also changes.Assuming that the variation of propagation time changing within theperiod of trigger signal T to be Δt_(d) and the echo signal intensity tobe A, then the echo signal E obtained by the n-th transmission/receptionis expressed by the following equation.

    E=A cos {ω(t.sub.d +n·Δt.sub.d)}      (1)

This is orthogonally detected, using the reference signal R (d) asmentioned below by the phase detector 104. The reference signal isavailable, though not shown, as V_(X) and V_(Y) which are uniform inamplitude but mutually different in phase by 90° from time t_(o) oftrigger signal T, which are expressed as follows. ##EQU1## By the phasedetector 104, the following signals are obtained by multiplying V_(X),V_(Y) of eq. (2) by E of eq. (1). ##EQU2## Two signals C (e) of eq. (3)obtained in the phase detector 104 are integrated in the integratingcircuit 108 by turning on the analog switch 107 in the period of t₁ tot₂ of gate signal G (f) generated by the gate signal generating circuit106. The signal C (e) expressed in eq. (3) is composed of two signalcomponents (ωnΔt_(d)) and (2ωt+ωnΔt_(d)), and the former does notcontain the parameter t of time and is a direct-current signal, whilethe latter is a high frequency signal with a frequency which is twice ashigh as the transmission frequency ω. Therefore, by passing through theintegrating circuit 108, the latter component is eliminated, and theDoppler shift signals X Y obtained as a result at the integration endtime t₂ are as follows.

    X=(A/2)·K.t.sub.G.cos (ωnΔt.sub.d)    (4)

    Y=-(A/2)·K·t.sub.G.sin (ωnΔt.sub.d) (4)

where K is a constant determined by the circuit constant of theintegrating circuit 108, and t_(G) is a time width of the gate signal G.The gate signal G is intended to match with the time t₁ to t₂ when theecho signal from a desired position of the body is received, and theintegration results X Y received the n-th time contain all of the echosignal information between t₁ and t₂ (g). The voltages of integrationresults X Y are held by the next sample and hold circuits 110 asindicated by the broken line (h) until the next n+1 time integrationresult is obtained. The integrating circuit 108 is reset when holding ofthe sample at hold circuits 110 is over.

Thus, the obtained integration result is a direct-current voltage, asclear from eq. (4), when transmitted and received only once, but sincethe transmission and reception are repeated every period T of thetrigger signal, n increases, and X Y continue to change while keeping aphase difference of 90°. This is an orthogonal Doppler signal, if, forexample, X is a real part signal and Y is an imaginary part signal. BothX and Y are obtained as discrete information at every period T, and thefollowing relationship is established between the delay time amountΔt_(d) varying in interval T from the n-th time transmission/receptionand n+1 time transmission/reception.

    Δt.sub.d =(2vT)/C                                    (5)

Therefore, from eqs. (4) and (5), the deviation frequency f_(d) ofDoppler shift signals X and Y is expressed as follows.

    f.sub.d =(2·v·f.cos θ)/C           (6)

where C is the speed of sound propagation in the body, f is thereference signal frequency (which is generally equal to the transmittedultrasonic pulse frequency), and θ is the angle formed by the movingdirection of the reflecting substance and ultrasonic wave runningdirection. In the body, when an echo signal from blood flow is captured,since the intensity of reflection by the blood flow is very small, theamplitude of Doppler shift signal is extremely weak, while the rate ofblood flow is fast, so that the deviation frequency f_(d) becomes high.In the body tissues such as viscera, the intensity of reflection A ishigh, and the movement is slow by the internal body movement or thelike, and the deviation frequency f_(d) is very low. By passing itthrough the high pass filter 111, only the Doppler shift signal from theblood flow having a small amplitude and high frequency can be obtained.To remove the Doppler shift signal from the body tissues is veryimportant for expanding the dynamic range of the frequency analyzer 112,and it is generally set at 100 to 1 kHz. In the frequency analyzer 112,the Doppler shift signals X and Y are analyzed in frequency, and thedirection of the blood flow is determined from the phase relationship ofthe two signals, and is displayed in the display unit 112 as a bloodflow pattern.

However, as compared with trigger signal T shown in FIG. 3(a), the echosignal E is roughly divided into the very intense portion b from thebody tissues appearing to be nearly standstill as shown in FIG. 3(b),and the feeble portion a from the blood flow which is always moving. Tomeasure the rate of blood flow, it is necessary to amplify the feebleecho signal portion a from the blood flow with a large gain, and withthe feeble echo signal from the blood flow alone, the real part signalXa and imaginary part signal Ya having only alternating-currentcomponents are obtained in the integrating circuit 108 in the amplitudeof up to amplitude limit V of the integrating circuit (c). When thesetting of the measuring point is at the intense echo signal portion bfrom the body tissue, the phase difference of reference signal R andecho signal E is always constant or changes very slowly, and at theoutput of the integrating circuit 108, the direct-current component orthe alternating-current component of the real part signal Xb and theimaginary part signal Yb with a very slow change appears (d). In theactual clinical application, since the ultrasonic beam transmitted fromthe probe 101 has a spread, if a blood vessel smaller in diameter thanthe beam diameter is captured, echo signals from the blood flow and bodytissue may often exist at the same time, and conventionally, the effectof the body tissue was eliminated by using a high pass filter 111.However, when a relatively shallow measuring point is observed, forinstance, the real part signal Xa and imaginary part signal Ya aresuperposed on the real part signal Xb and imaginary part signal Yb,resulting in Xc and Yc (e). But because of the amplitude limit V of theintegrating circuit 108, the portion indicated by shaded area clips, andthe waveform is distorted. When the direct-current component is smalland saturation is not caused, it is possible to remove thedirect-current component by the conventional high pass filter 111, butwhen measuring the blood flow in relatively superficial carotid arteryor deep-seated fine blood vessels, the echo signal from the body tissuearound the blood vessel is particularly intense due to spread of theultrasonic beam, and the direct-current component increasesparticularly. As a result, the integrating circuit 108 is saturated bythis direct-current component alone, and Doppler shift signal does notappear. Therefore, if the direct-current component is removed by thehigh pass filter 111, a Doppler shift signal cannot be obtained, and theresult of frequency analysis involves unnecessary frequency components,or the result of analysis is suspended. Or, if either Doppler shiftsignal X or Y is saturated, information about the direction of bloodflow cannot be obtained, which was a serious problem clinically.

The change of time t_(G) while gate signal G is ON influences theshut-off frequency fc, and the low frequency component decreases as thegate width is wider, so that the obtained blood flow information variesdepending on the gate width even at the same location. Generally, thefrequency of the high pass filter 111 is set by the physician before usedepending on the diagnostic condition, and the set frequency is shown onthe operation panel or display unit of the apparatus, but the shut-offfrequency fc of the integrating circuit affects the shut-off frequencyof the high pass filter, and the displayed set frequency is no longercorrect. Thus, depending on the gate width, the obtained informationdiffers, and the displayed set frequency is not correct, and theaccuracy as diagnostic data is lowered, and the reproducibility isinferior, and the result of diagnosis may differ depending on the gatewidth, which posed serious problems.

SUMMARY OF THE INVENTION

This invention is to solve the above-discussed problems of the priorart, and it is hence a primary object of this invention to present anultrasonic Doppler blood flowmeter capable of analyzing the frequency ofthe blood flow only and obtaining an accurate blood flow information, byremoving the direct-current components generated by the echo signals ofbody tissues from the real part output and imaginary part outputdelivered from the integrating circuit.

It is another object of this invention to present an ultrasonic Dopplerblood flowmeter which is capable of diagnosing accurately by using theblood flow information and high pass filter characteristics and which iscompletely free from effects of the gate width.

These and other objects of this invention may be achieved by anapparatus which comprises a phase detector for detecting the phase ofecho signals including Doppler shift from the blood flow in the body ofa subject, a reference signal generating circuit to serve as thereference for the frequency and phase of the reference signal whendetecting the phase of transmission signal from a drive circuit whichdrives an ultrasonic probe and the above echo signals, a gate signalgenerating circuit for generating a gate signal at the timingcorresponding to the echo signal from an intended portion, a means forcontrolling the opening and closing of the phase signal detected by saidphase detector by said gate signal, an integrating circuit for obtaininga Doppler shift signal by repeating transmission and reception byintegrating the phase signal passing through said control means in theperiod of the above gate signal alone and determining the sum of thephases within the period of gate signal, and a direct-current feedbackcircuit which negatively feeds back the direct-current component orultralow frequency component of the Doppler shift signal obtained insaid integrating circuit into this integrating circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram showing a conventional Doppler bloodflowmeter;

FIG. 2 is a time chart showing the operation of the flowmeter of FIG. 1;

FIG. 3 is an integrated waveform obtained by using the flowmeter of FIG.1;

FIG. 4 is a block diagram representing one of the embodiments of anultrasonic Doppler blood flowmeter according to this invention;

FIG. 5(a) is a practical circuit diagram in an embodiment of theessential parts of the flowmeter of FIG. 3, FIG. 5(b) is a time chartshowing the operation of the circuit shown in FIG. 5(a); FIG. 5(c) is apractical circuit diagram in a different embodiment of the essentialparts of the flowmeter of FIG. 3, FIG. 5(d) is a time chart showing theoperation of the circuit shown in FIG. 5(c); and FIG. 5(e) is a diagramshowing the frequency characteristics of the circuits shown in FIGS.5(a) and 5(c);

FIG. 6 is a diagram used to compare the integrated waveforms of thisinvention and the prior art;

FIG. 7 is a block circuit diagram showing an ultrasonic Doppler bloodflowmeter in a different embodiment of this invention;

FIGS. 8(a) and 8(b) the shows frequency characteristics ofdirect-current feedback circuit and integrating circuit of the flowmeterof FIG. 7; FIG. 8(a) is a frequency characteristic diagram of thedirect-current feedback circuit and FIG. 8(b) is a frequencycharacteristic diagram of the integrating circuit with negative feedbackapplied;

FIGS. 9(a) and 9(b) are practical circuit diagrams showing theintegrating circuit, direct-current feedback circuit, and feedback gaincontrol circuit in a further different embodiment of the ultrasonicDoppler blood flowmeter of this invention; and

FIG. 10(a) is a practical circuit diagram showing the integratingcircuit, direct-current feedback circuit, and feedback gain controlcircuit in a still further embodiment of the ultrasonic Doppler bloodflowmeter of this invention, and FIG. 10(b) is an operation timing chartof the circuit of FIG. 10(a).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 is a block diagram of an ultrasonic Doppler blood flowmeter inone of the embodiments of this invention. The ultrasonic Doppler bloodflowmeter of this invention possesses a direct-current current feedbackcircuit 9 for negatively feeding back the direct-current and ultraslowfrequency components of the orthogonal signal delivered by theintegrating circuit or sample and hold circuit in the above conventionalultrasonic Doppler blood flowmeter into the input of the integratingcircuit of way of an analog switch.

That is, numeral 1 is an ultrasonic transmission and reception means(hereinafter called a probe) which transmits an ultrasonic pulse intothe body from its ultrasonic transmission/reception plane 1a, andreceives the echo signal reflected due to difference in the acousticimpedance. The probe 1 is generally made of a piezoelectric material.The probe 1 may be either a transmission/reception combination type ortransmission and reception separate type. Numeral 2 denotes a drivecircuit which drives the probe 1 by generating the drive voltage forgenerating ultrasonic pulses transmitted from the probe 1 at thefrequency of an external clock and at a timing of an external trigger.Numeral 3 denotes a transmission timing circuit which provides thetiming for the drive circuit 2 to generate the drive voltage as atrigger. Numeral 4 denotes a phase detector which detects the phase ofecho signals received by the probe 1. Numeral 5 denotes a referencesignal generating circuit which serves as the reference for frequencyand phase of the reference signal when detecting the phase of thetransmission signal and echo signal of the drive circuit 2 by means ofthe phase detector. Numeral 6 denotes a gate signal generating circuitwhich generates a gate signal at a time corresponding to the propagationtime of the ultrasonic wave up to the transmission/reception plane 1a ofthe probe 1 and an intended position. Numeral 7 denotes an analog switchwhich causes the phase signal and direct-current feedback voltagedetected by the phase detector 4 to pass through during the period ofthe gate signal generated by the gate signal generating circuit 6.Numeral 8 denotes an integrating circuit which integrates the phasesignal passing through the analog switch 7, determines the sum of phasesignals, repeats transmission and reception, and obtains Doppler shiftsignals. Numeral 10 denotes a sample and hold circuit which holds theintegrated result for resetting until the next integration result isobtained prior to integration by the integrating circuit 8. Numeral 9denotes a direct-current feedback circuit which negatively feeds backthe direct-current component or ultralow frequency component of Dopplershift signal delivered from the integrating circuit 8 or sample and holdcircuit 10 to the integrating circuit 8 by way of the analog switch 7.Numeral 11 denotes a high pass filter which removes signals of less thanseveral hundred hertz from the Doppler shift signal obtained in theintegrating circuit 8. Numeral 12 denoted a frequency analyzer whichanalyzes the frequency of the Doppler shift signal passing through thehigh pass filter 11. Numeral 13 denotes a display unit which shows theresult of the frequency analyzer 12.

The operation of this embodiment is described below. In this embodiment,too, the echo signal is orthogonally detected according to the routeexplained in the conventional example in FIGS. 1 and 2, and is displayedafter frequency analysis. Here, the integrating circuit 8 outputs arenegatively fed back by the direct-current components of the real partsignal X and the imaginary part signal Y after integration, and it worksas high pass filter in ultralow frequency region.

FIGS. 5(a) and 5(c) depict examples of practical circuits of the analogswitch 7, integrating circuit 8, sample and hold circuit 10, anddirect-current feedback circuit 9 as shown the embodiment of FIG. 4, andFIGS. 5(b) and (d) denote signal waveforms at each terminal of FIGS.5(a) and (c). The circuit in FIG. 5(a) or FIG. 5(c) requires one systemfor the real part signal X and one system for the imaginary part signalY, but only one system is shown in FIG. 5 the diagram. The circuits in(a) and (c) produce exactly the same effect. As a first embodiment, theoperation of the circuit in FIG. 5(a) is explained below while referringto FIG. 5(b). The integrating circuit 8 in FIG. 4 is composed of, asshown in FIG. 5(a), an inverting amplifier OP₁ having a degree ofamplification of -A₁, a resistance R₁, and capacitor C₀, and it has twoinput terminals, that is, E_(i) to which the output of the phasedetector 4 (see FIG. 4) is applied, and E_(f).sbsb.o to which adirect-current feedback voltage of direct-current feedback circuit 9 isapplied, and the signals of both terminals are added immediately beforethe analog switch 7, and are integrated when the analog switch 7 is ON.The analog switch 7 is turned on in the period of t₁ to t₂ of gatesignal G, and the signals are integrated for the time t_(g), and awaveform as shown in the drawing is obtained at the input terminal(output of integrating circuit) E_(f).sbsb.i of the direct-currentfeedback circuit 9. The sample and hold circuit 10 has a gain of -A, andsamples from t₂ when the integration is over to time t.sub.ω, and holdsthe sample after t₃. After holding, the integrating circuit, prior tothe next transmission and reception, turns on the analog switch 7' todischarge C₀ for the period of t₃ to t₄ by means of RESET signal, and isreset. At the output terminal E.sub. 0 of the sample and hold circuit10, a Doppler signal which has a discrete value in period T appears asshown in the diagram, and is supplied to high pass filter 11 (see FIG.4). On the other hand, the direct-current feedback circuit 9 comprises alow pass filter by means of inverting amplifier OP₃, capacitor C_(f) andresistance R_(f), and direct-current and ultralow frequency componentsare extracted from the output signal of the integrating circuitE_(f).sbsb.i, and are negatively fed back through resistance R₂. Theinverting amplifier OP₂ has a gain of -A_(f), and is intended to invertthe phase. Therefore, the lower the frequency of the output of theintegrating circuit, the more increases the feedback amount, and theintegration result becomes smaller. Such input, output characteristicsof the circuit may be obtained as follows. ##EQU3## Equation (7) refersto the relationship between the mean voltage _(i) of output of phasedetector within gate time t_(g) supplied to input terminal E_(i),direct-current feedback voltage e_(f).sbsb.0 supplied to input terminal_(f).sbsb.0, and integrating circuit output _(f).sbsb.i, and eq. (8)refers to the integrating circuit output _(f).sbsb.i and direct-currentfeedback voltage e_(f).sbsb.0 supplied to the direct-current feedbackcircuit 9. In eq. (8), ω is the frequency of Doppler shift signaldelivered to the integrating circuit, and (t_(g) /K+t.sub.ω)/T is theduty cycle ratio of the Doppler shift signal existing discretely inevery period T, and during the integration time of t₁ to t₂, its valueis less than 1 to correct the duty cycle ratio in this duration becauseit is in the transient state until _(f).sbsb.i. From eqs. (7), (8), thegain A_(v) of circuit in FIG. 5(a) becomes as follows. ##EQU4##Incidentally, the -3 dB low range cut-off frequency f_(c) in eq. (9) isexpressed as follows. ##EQU5##

The value of f_(c) should be selected at a frequency to sufficientlyallow the passage of the Doppler shift signal due blood flow and toinhibit the Doppler shift signal due to internal movement of the body;in an ordinary ultrasonic pulse Doppler blood flowmeter with anultrasonic pulse frequency of about 2 to 7 MHz, it should be properlyselected to be in the range of 50 to 500 Hz.

As a second embodiment, the operation of the circuit in FIG. 5(c) isexplained below while referring to FIG. 5(d). The principal constructionand operation are same as in the first embodiment explained in FIG.5(a), except that the output of the sample and hold circuit 10 issupplied to the integrating circuit, as compared with the firstembodiment in FIG. 5(a) wherein the output of the integrating circuit issupplied to the input of the integrating circuit through the analogswitch 7 by the direct-current feedback circuit 9. Incidentally, theinverting amplifier OP₂ used in the first embodiment is not neededbecause the sample and hold circuit 10 fulfills this function in thisembodiment. The input and output characteristics of this circuit areobtained as follows. ##EQU6##

Equations (11) and (12) correspond respectively to eqs. (7) and (8),from which the gain A_(v) of the circuit in FIG. 5(c) is expressed asfollows. ##EQU7##

Meanwhile, the -3 dB low range cut-off frequency f_(c) in eq. (13) isexpressed in the following equation. ##EQU8##

The value of f_(c) is determined in the same fashion as in FIG. 5(a),and FIG. 5(e) illustrated the frequency characteristics of the circuitsin FIG. 5(a) and FIG. 5(c). As shown in the diagram, only the effects ofthe body tissues can be eliminated, and a frequency f_(c) free fromadverse effects on the detection of Doppler shift from the blood flowcan be selected, so that only the alternating-current components whichare Doppler shift signals can be delivered to the subsequent circuitswithout integrating the direct-current components. Thus, the Dopplershift signals appearing at the orthogonal signals X and Y, being rid ofdirect-current components, are completely free from influences ifsignals from the body tissues are received simultaneously in the echosignals.

FIG. 6 shows the output waveforms of the integrating circuit 8 of thisembodiment and the prior art. The solid line refers to this invention,and the broken line denotes the prior art, and the shaded area is theportion of the waveform being cut off by saturation. As shown in thediagram, by executing this invention, the amplitude limit is expanded,and there occurs no effect of direct-current voltage, so that a veryfine blood vessel can be measured, without saturating the integratingcircuit 8, and that the amplitude of the Doppler shift signal can makeuse of the amplitude limit V to the maximum extent.

As explained above, by the ultrasonic Doppler blood flowmeter of thisinvention, since the direct-current component or ultralow frequencycomponent of Doppler shift signal obtained from the integrating circuitis negatively fed back by the direct-current feedback circuit, if anecho signal without a time change from the body tissue is captured atthe same time as an echo signal from the blood flow, in this simplecircuit composition, the frequency of the blood flow only can beanalyzed, and no display other than that of the blood flow appears onthe blood flow pattern on the display unit, so that accurate informationabout the direction of the blood flow can be obtained.

FIG. 7 is a block circuit diagram of an ultrasonic Doppler bloodflowmeter in another embodiment of this invention. This embodimentinvolves a feedback gain control circuit 14 which controls the feedbackgain of the direct-current feedback circuit 9 in the composition shownin FIG. 4, depending on the gate width information set in the gatesignal generating circuit 6. The other structure and operation are thesame as that of FIG. 4.

The high pass filter of integrating circuit 8 has the characteristic asshown in FIG. 8(b), but by expanding the gate time t_(G), the durationof the application of the feedback signal to the integrating circuit 8is extended, and the same effect as when the gain of the direct-currentfeedback circuit 9 is increased is produced, which causes to shift thecharacteristic 1 in FIG. 8(a) toward the direction P to becomecharacteristic 2, which results in the characteristic 2 of theintegrating circuit FIG. 8(b). To keep the frequency characteristic ofthe integrating circuit 8 at curve 1, as the gate width expands, thegain of the direct-current feedback circuit 9 is lowered to correct inthe direction of P'. Or, by lowering the cut-off frequency of thedirect-current feedback circuit 9 and correcting the characteristic inthe direction of Q', the increase of the gain of direct-current feedbackcircuit 9 due to the expansion of the gate width is canceled, so thatthe cut-off frequency f_(c) may be kept constant. When the gate width isshortened, similarly, it is enough to correct the characteristic of thedirect-current feedback circuit 9 in the direction of P or Q. Thefeedback gain control circuit 14 is responsible for such correction, andit controls the degree of amplification A_(f) of the entiredirect-current feedback circuit 9, corresponding to the set gate width,that is, the time t_(G) while analog switch 7 is ON, by determining theamplification according to the following equation.

    Afocl/t.sub.G                                              (15)

FIGS. 9(a) and (b) relate to an embodiment of circuit structure forcorrecting the above-mentioned shut-off frequency, in which FIG. 9(a)shows a circuit which is designed to correct the gain of thedirect-current feedback circuit 9 by means of a variable gain amplifier9a according to eq. (15), while FIG. 9(b) shows a circuit which changesthe cut-off frequency of the direct-current feedback circuit 9, bychanging the resistance Rf. In FIG. 8(a), when the gate width isextended, the characteristic 1 moves toward the direction of P, and itis corrected by moving to the direction of Q', by lowering the cut-offfrequency. Besides, it is also possible to correct by varying thecapacitor Cf or resistance R₂.

FIG. 10(a) refers to a further different embodiment, in which 14a is ananalog switch used to turn on and off the Doppler shift signal fed intothe direct-current feedback circuit 9; the feedback gain controlscircuit 14 control the time width t_(f) in which the analog switch 14ais ON by the gate width t_(G). FIG. 10(b) shows the control timing ofthe analog switch 14a. Its operation is described below. As in thepreceding embodiments, the Doppler shift signal appearing at output E₀has its ultralow frequency components in the direct-current feedbacksignal 9, and is fed back in reverse phase, but at the input ofdirect-current feedback circuit, the analog switch 14a is connected, andby controlling the duty ratio t_(f) /T of the gain control signal G_(f)generated depending on the gate width t_(G) by the feedback gain controlcircuit 14, the entire gain of the direct-current feedback circuit 9 isvaried and corrected.

Thus, by the ultrasonic Doppler blood flowmeter of these embodimentsherein, by applying a gain depending on the gate width to thedirect-current feedback circuit by the feedback gain control circuit,the cut-off frequency of the integrating circuit may be kept constantregardless of the gate width, and the measurement of the rate of bloodflow being free from the effects of the Doppler shift signal from thebody tissues can be effected in the same conditions regardless of thegate width, and its clinical effect is extremely notable.

What is claimed is:
 1. An ultrasonic Doppler blood flowmeter adapted formeasuring the flow of blood through a body of a subject, said flowmetercomprising, at least, an ultrasonic transmission means for transmittingultrasonic pulses into the body of a subject; an ultrasonic receptionmeans for receiving echo signals reflected within the body; a drivecircuit means for generating ultrasonic pulses to be transmitted by saidultrasonic transmission means; a transmission timing signal circuitmeans for generating timing signals used to generate ultrasonic pulsesby means of said drive circuit means; a phase detecting means fordetecting the amplitude and phase of each of said echo signals obtainedby said ultrasonic reception means; a reference signal generationcircuit means for generating a reference signal serving as the referencefor the frequency and phase of a transmission signal of said drivecircuit means and a reference signal for frequency and phase whendetecting the phase of each of said echo signal; a gate signalgenerating circuit means operatively connected to said transmissiontiming signal circuit means and to said reference signal generatingcircuit means for generating a gate signal of variable period at a timecorresponding to an echo signal from a desired position; a control meansfor controlling the transmission therethrough of the phase signaldetected by said phase detecting means in response to said gate signal;an integrating circuit means for obtaining a Doppler shift signal byintegrating the phase signal transmitted through said control means; asample and hold circuit means for temporarily holding the integratedresult obtained by said integrating circuit means; a variable gainfeedback circuit means for negatively feeding back a direct-currentcomponent of an ultra-low frequency component of the Doppler shiftsignal obtained by said integrating circuit means or held by said sampleand hold circuit into said integrating circuit means; and a feedbackgain control circuit means for controlling the gain of said feedbackcircuit in proportion to the period of said ultrasonic pulses or ininverse proportion to the time width of said gate signal generated bysaid gate signal circuit means.
 2. An ultrasonic Doppler blood flowmeteras recited in claim 1, wherein said feedback circuit means is consistingof a low pass filter using an integrating circuit means and a gainvariable amplifier for controlling the gain thereof.
 3. An ultrasonicDoppler blood flowmeter as recited in claim 1, wherein said feedbackcircuit means is consisting of a low pass filter using an integratingcircuit means and a time constant control circuit means for controllingthe time constant of said integrating circuit means.
 4. An ultrasonicDoppler blood flowmeter as recited in claim 1, wherein said feedbackcircuit means is consisting of a low pass filter using an integratingcircuit means and a chopper circuit means which turns the input of saidintegrating circuit means on and off.
 5. An ultrasonic Doppler bloodflowmeter as recited in claim 1, wherein said feedback circuit means isconnected to said control means such that said feedback signal from thefeedback circuit means is fed back to the input terminal of said controlmeans.
 6. An ultrasonic Doppler blood flowmeter as recited in claim 1,wherein said ultrasonic transmission means and said ultrasonic receptionmeans together comprise one ultrasonic probe.
 7. An ultrasonic Dopplerblood flowmeter as recited in claim 1, wherein said control meanscomprises an analog switch.